DE102015108504B4 - Method for safely booting a computer system and computer system - Google Patents

Abstract

Method for safely booting a computer system (10) having a main processor (11), a crypto processor (12), a multiplexer (14) and an SPI module (13), wherein in the SPI module (13) the program code of the at least one Bootloaders is stored and to the multiplexer (14) of the main processor (11), the crypto processor (12) and the SPI module (13) are electrically connected, the method comprising the steps of: - switching the multiplexer (14) by the Crypto processor (12) so as to establish an electrical connection from the SPI device (13) to the main processor (11); executing program code of at least one bootloader to load a kernel after the step of switching the multiplexer, the kernel passing through switching the multiplexer (14) by the crypto-processor (12) after the step of executing the program code of the at least one bootloader so that an electrical connection of the SPI module (13) is set up to the crypto processor (12) and the electrical connection between the SPI module (13) and the main processor (11) is interrupted, - checking the program code of the bootloader by the crypto processor (12) after an electrical Connection from the SPI device (13) to the crypto processor (12) has been established after at least a portion of the program code of the bootloader has been executed; and - interrupting the booting by the crypto processor (12) if a result of the checking of the program code of the boot loader indicates a manipulation of the program code of the boot loader.

Description

The invention relates to a method for safely booting a computer system and to a computer system with a device for checking the program code of a boot loader.

In computer systems with safety-critical applications, it is necessary to already check and secure the booting of the computer system. Such computer systems are used, for example, to authenticate or to perform payment transactions from users. A user can authenticate himself by entering codes or biometric data. In order to make it difficult for unauthorized third parties to access the computer system and thus gain access to personal data, booting the computer system checks the software, firmware and the processes of booting itself. Thus, it can be ensured that secure and original firmware and hardware execute booting within the computer system.

The publication EP 1 953 666 A2 describes a method of booting an electronic device and a method of authenticating the booting of an electronic device.

From the WO 2014/175 862 A1 a redundant system boot code is known in a second nonvolatile memory.

It is an object of the invention to provide an improved method of operating such a computer system and an improved computer system.

• - Ausführen von Programmcode wenigstens eines Bootloaders zum Laden eines Kernels;
• - Überprüfen des Programmcodes des Bootloaders, nachdem wenigstens ein Teil des Programmcodes des Bootloaders ausgeführt wurde; und
• - Unterbrechen des Bootens, wenn ein Ergebnis des Überprüfens des Programmcodes des Bootloaders auf eine Manipulation des Programmcodes des Bootloaders hinweist.

According to one aspect of the invention, the object is achieved by a method for safely booting a computer system. The method comprises the steps:

• Executing program code of at least one boot loader for loading a kernel;
• - checking the program code of the bootloader after at least a part of the program code of the bootloader has been executed; and
• - Suspend booting if a result of checking the program code of the boot loader indicates a manipulation of the program code of the boot loader.

The computer system includes a main processor and a crypto processor. This loads the kernel through the main processor. The check is performed in the crypto processor. The crypto processor also executes booting, particularly by sending a reset signal to the main processor.

• - Schalten des Multiplexers durch den Cryptoprozessor, so dass eine elektrische Verbindung von dem SPI-Baustein zu dem Hauptprozessor aufgebaut wird.

The computer system additionally comprises a multiplexer and an SPI module. The program code of the at least one boot loader is stored in the SPI module. At the multiplexer, the main processor, the crypto processor and the SPI module are electrically connected. Before the execution step, the following additional step is performed:

• Switching the multiplexer through the crypto processor so that an electrical connection is established from the SPI module to the main processor.

• - Schalten des Multiplexers durch den Cryptoprozessor, sodass eine elektrische Verbindung von dem SPI-Baustein zu dem Cryptoprozessor aufgebaut wird.

The crypto processor additionally performs the following step after the step of executing the program code of the at least one boot loader, before the step of checking is executed:

• Switching the multiplexer through the crypto processor so that an electrical connection is established from the SPI module to the crypto processor.

In this case, the electrical connection between the SPI module and the main processor is interrupted.

The check of the program code of the boot loader by a crypto processor leads to a high safety standard. The review by the crypto processor is performed in parallel with the booting of the computer system. This speeds up booting compared to booting with a previous check.

After a power-on signal, the crypto processor configures the multiplexer to establish a connection between the SPI device and the main processor. This allows the main processor to load the program code of the at least one bootloader from the SPI module.

For the verification step, an electrical connection is made between the crypto processor and the SPI device. This is built by switching the multiplexer through the crypto processor. In this case, an electrical connection between the SPI module and the main processor is interrupted. The main processor executes the kernels while checking the program code of the boot loader. The kernel does not need to access the SPI device. Thus, the crypto processor can check the contents of the SPI device at the same time. Subsequently, a connection between SPI module and the Main processor are restored by the crypto processor.

After booting the computer system, a bootloader is executed. The bootloader initiates the loading of a kernel of the computer system. The program code of the bootloader is checked after at least a part of the program code of the bootloader has been executed. Checking the boot loader after a part of the program code of the bootloader has already been executed can boot the computer system faster than if the program code of the bootloader were checked before the bootloader is executed.

If the check detects that the program code of the bootloader has been tampered with, the boot process is interrupted. Thus, the security of the computer system is maintained since the computer system does not boot completely without performing a check on the boot loader.

In this context, booting includes the operation in the computer system from a turn-on signal to a normal operating state.

According to a further advantageous embodiment, the step of checking is carried out after completing an execution of the program code of the boot loader and loading at least part of the kernel.

The program code of the boot loader is stored in any memory that can be addressed separately, such as a non-volatile memory device, in particular an SPI device. After loading the kernel, the main processor does not access the memory of the program code of the boot loader, so that the check can be performed by the crypto processor independent of the main processor.

According to a further advantageous embodiment, the step of checking is carried out after a predetermined time has elapsed after the loading of the kernel has been started.

In order to ensure a smooth timing of the check in the boot process, a timing that starts with starting the loading of the kernel, can be set at what time the check should take place.

According to a further advantageous embodiment, the step of interrupting comprises stopping the main processor.

Stopping the main processor will interrupt the boot process. The computer system does not come into a normal operating state in which malicious software would be executable. Any malware already started will be aborted as well as the boot process itself. In particular, the main processor is stopped by a reset. For example, the supply voltage of the main processor is interrupted.

According to a second aspect of the invention, the object is achieved by a computer system having a memory for a program code of a boot loader, a main processor and a device for checking the program code of the boot loader. Here, the main processor is set up to run the boot loader and load a kernel. The device is electrically coupled to the memory and the main processor. The device is adapted to perform a check of the program code of the boot loader after at least a part of the program code of the boot loader has been executed. Furthermore, the device is set up to interrupt a booting of the computer system if the check of the program code of the boot loader indicates a manipulation of the program code of the boot loader.

The device for checking the program code of the boot loader comprises a crypto processor.

The computer system has an SPI module in which the memory is arranged.

The electrical coupling of the device to the memory and the main processor comprises a multiplexer.

Such a computer system can boot quickly, while maintaining a high level of security.

The crypto processor is adapted to drive the multiplexer to establish an electrical connection between the SPI device and the crypto processor and to interrupt the electrical connection between the SPI device and the main processor after the kernel load has been started and before the cryptoprocessor begins to check the program code stored in the SPI block.

Advantages with respect to the second aspect will be apparent from the advantages with respect to the first aspect as well as the embodiments described below.

• 1 ein Computersystem gemäß einer Ausgestaltung der Erfindung;
• 2 ein Ablaufdiagramm eines Verfahrens gemäß einer Ausgestaltung der Erfindung; und
• 3 eine weitere Ausgestaltung des Computersystems.

The invention will be described below with reference to figures. In the figures show:

• 1 a computer system according to an embodiment of the invention;
• 2 a flowchart of a method according to an embodiment of the invention; and
• 3 another embodiment of the computer system.

1 shows a schematic representation of a computer system 10 , In the illustrated embodiment, the computer system 10 a payment terminal with which a user can make electronic payments. In other embodiments, the computer system may be 10 For example, to act a mobile phone, a tablet PC or a desktop PC. Of course, other types of computer systems are conceivable.

The computer system 10 has a main processor 11 on. The main processor 11 serves to processes the computer system 10 perform. For example, an operating system is running on the main processor 11 from. In the exemplary embodiment, it is the main processor 11 a so-called system on a chip. This is the main processor 11 various functions, which may be distributed to various components, for example, in other embodiments, such as a desktop PC.

The computer system 10 further includes a memory device having a serial peripheral interface (SPI). This The SPI building block 13 in the exemplary embodiment comprises a memory area, is stored on the program code of a boot loader. The bootloader here comprises a bootloader of a first level and a bootloader of a second level. After a turn-on signal from the computer system 10 and initialization of the first stage boot loader, the second stage boot loader is automatically started by the first level boot loader and then executed. The task of the bootloader is to use the computer system 10 to prepare for a loading of the operating system and thus to a normal operating state. In particular, booting includes performing a system bootstrap, a first level bootloader, and a second level boot loader, as well as loading a kernel, an initialization file system, and an initram file system.

Through the main processor 11 For example, a kernel is loaded to run an operating system. The loading of the kernel is initialized by the bootloader of the second level. The main processor loads the kernel from another memory connected to the main processor. In another embodiment, the main processor loads the kernel from the SPI device 13 ,

In the embodiment, the main processor 11 and the SPI building block 13 via a multiplexer 14 connected. In addition, at the multiplexer 14 a crypto processor 12 connected. In the preferred embodiment, the crypto processor is 12 a chip that has a high security standard and whose firmware can be programmed for security checks. The crypto processor 12 can the multiplexer 14 drive. In particular, the cryptoprocessor 12 a connection of the SPI module 13 from the main processor 11 to the crypto processor 12 switch. Here then either the crypto processor 12 or the main processor 11 with the SPI module 13 electrically coupled.

The crypto processor 12 is further with the main processor 11 electronically coupled, so the crypto processor 12 at least one reset signal to the main processor 11 can send.

2 shows a flowchart for a method for operating the computer system 10 according to the embodiment described above. After starting the computer system 10 controls the crypto processor 12 the multiplexer 14 such that the multiplexer establishes an electrical connection between the SPI device 13 and the main processor 11 manufactures.

In step 22 the bootloader is loaded and executed. This includes loading and running the first-level bootloader and the second-level boot loader. In this case, the second-stage boot loader is loaded by the first-stage boot loader because the first-stage boot loader memory is too small to address and initialize all peripherals.

After running the bootloader in step 22 will be in the main processor 11 loading the kernel in step 23 started. In the embodiment, the main processor accesses 11 while loading the kernel, not on the SPI device 13 to.

Because the SPI building block 13 not from the main processor 11 can be addressed while the kernel loads, the crypto processor 12 on the SPI module 13 access and carry out the safety-relevant check of the program code of the bootloader. The crypto processor controls this 12 in step 24 the multiplexer so that between the SPI module 13 and the crypto processor 12 an electrical connection is made. The electrical connection between the SPI building block 13 and the main processor 11 is interrupted here.

In the embodiment, the crypto processor switches 12 the multiplexer 14 in step 24 five seconds after starting to load the kernel in step 23 around. In an alternative embodiment, the crypto processor switches in step 24 the multiplexer after a time lapse, immediately from the first switching of the multiplexer in step 21 depends. Here, the boot loader is provided with a predetermined period of time to initialize the loading of the kernel.

The crypto processor 12 now starts in step 25 the program code contained in the SPI building block 13 saved, check. In particular, the program code of the boot loader, or the boot loader of the first stage and / or the boot loader of the second stage is checked.

This is done by the cryptoprocessor 12 from the memory data of the SPI block 13 a hash value that the crypto processor does 12 checked against a reference value. For this purpose, the reference value or a comparison table with the reference value is stored in a secure memory of the crypto processor. In another embodiment, the reference value is stored in an external memory connected to the crypto processor.

If the check leads to a positive result, it can be assumed that the program code of the boot loader is in order. The computer system 10 sets the boot up to step 26 thus stopping the boot without interruption by the crypto processor 12 , This loads the main processor 11 complete the kernel and then start an operating system. The fact that the boot process is not affected by the cryptoprocessor is indicated by the dashed arrow in 2 indicated.

Is the result of the review in step 25 negative, this indicates a manipulation of the program code of the boot loader in the SPI block 13 out. The crypto processor 12 send in step now 27 a reset signal to the main processor 11 , The reset signal turns on the power supply of the main processor 11 interrupted and the boot process stopped immediately. In the embodiment, the reset signal becomes the main processor 11 via a GPIO (general purpose input / output) pin of the crypto processor 12 Posted.

Together with the setting of the reset signal, the crypto firmware of the crypto processor can additionally set a marker in a so-called tamper register. As a result, it is deposited that a manipulation has been detected. A status signal, such as an LED display, can be used to emit a warning signal. The status indicator is on both the main processor 11 as well as the crypto processor 12 connected. By setting the reset signal on the main processor 11 he is without voltage and has no access to the status display. Thus, the status indicator is from the crypto processor 12 controlled. This ensures that the status indicator that indicates a manipulation is also from the crypto processor 12 is controlled.

An alternative, in 3 illustrated method for a safe and fast booting of the computer system 10 it is to ensure an SPI module in the crypto processor 32 to emulate. This is indicated by a computer system 30 no physical SPI device on which a bootloader is stored. Likewise, the computer system 30 no multiplexer. The crypto processor 32 is electronically directly with a main processor 31 coupled.

If the crypto processor receives a switch-on signal, the crypto-processor starts 32 an emulation of an SPI device in the crypto processor 32 , This is a safe storage area 33 used. In the safe storage area 33 For example, it is a secure EEPROM. The secure storage area 33 is via a secure connection to the crypto processor 32 connected, on the only the Cryptoprozessor 32 Has access. In an alternative embodiment, the secure memory area is located 33 within the crypto processor 32 ,

By implementing the emulation of an SPI module in the safe memory area 33 of the crypto processor 32 is a secure anchor for starting a secure code checking in the boot loader of the first stage created. From this secure anchor further program code can be executed safely. The crypto processor 32 and those on the crypto processor 32 stored firmware is considered a trusted element and thus trusted. A programming of the in the safe memory area 33 stored Bootloaders, or the program code of the boot loader can only via a secure algorithm of the crypto processor 32 take place (Flashalgorithmus). Thus it is guaranteed that no unauthorized third party program code on the Cryptoprozessor 32 play.

After the emulation of the memory module can from the Cryptoprozessor 32 out the boot loader, especially the first stage boot loader and the second stage boot loader. Because the emulation of the SPI building block 13 in the crypto processor 32 takes place, no check of the program code of the boot loader is necessary. The computer system 30 can thus directly Durchbooten and requires no interruption or verification of the program code of the boot loader.

LIST OF REFERENCE NUMBERS

10, 30

computer system

11, 31

main processor

12, 32

Crypto processor

13

SPI device

14

multiplexer

33

secure storage area

21 - 27

steps

Claims (5)

Method for safely booting a computer system (10) having a main processor (11), a crypto processor (12), a multiplexer (14) and an SPI module (13), wherein in the SPI module (13) the program code of the at least one Bootloaders is stored and to the multiplexer (14) of the main processor (11), the Cryptoprozessor (12) and the SPI module (13) are electrically connected, the method comprising the following steps: - switching the multiplexer (14) by the crypto processor (12) so that an electrical connection is established from the SPI module (13) to the main processor (11); Executing program code of at least one boot loader for loading a kernel after the step of switching the multiplexer, the kernel being loaded by the main processor (11); Switching the multiplexer (14) by the crypto-processor (12) after the step of executing the program code of the at least one bootloader so as to establish an electrical connection from the SPI device (13) to the crypto-processor (12) and establish the electrical connection between the SPI module (13) and the main processor (11) is interrupted; - checking the program code of the boot loader by the crypto processor (12) after establishing an electrical connection from the SPI module (13) to the crypto processor (12) after at least a part of the program code of the bootloader has been executed; and - interrupting the booting by the crypto processor (12) if a result of the checking of the program code of the boot loader indicates a manipulation of the program code of the boot loader. Method according to Claim 1 wherein the step of checking is performed after completing an execution of the program code of the boot loader and loading at least part of the kernel. Method according to one of Claims 1 or 2 wherein the step of checking is performed after a lapse of a predetermined time after the kernel load has been started. Method according to one of Claims 1 to 3 wherein the step of interrupting comprises stopping the main processor (11). A computer system (10) having a memory for a program code of a boot loader, a main processor (11) and a device for checking the program code of the boot loader, wherein the main processor (11) is adapted to execute the bootloader and load a kernel, and wherein the Device is electrically coupled to the memory and the main processor (11) and is adapted to perform a check of the program code of the boot loader after at least a part of the program code of the boot loader has been executed, and to interrupt the booting of the computer system (10), if a The result of checking the program code of the boot loader indicates a manipulation of the program code of the boot loader, wherein the device for checking the program code of the boot loader comprises a crypto processor (12), the computer system (10) having an SPI block (13) in which the Memory is arranged, wherein the electrical coupling of the Device with the memory and the main processor (11) comprises a multiplexer, wherein the crypto-processor (12) is adapted to control the multiplexer so that between the SPI module (13) and the crypto-processor (12) made an electrical connection and the electrical connection between the SPI device (13) and the main processor (11) is interrupted after the loading of the kernel has been started and before the crypto processor (12) checks the program code stored in the SPI device (13) starts.

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